Unmodified polystyrene resin, often called crystal polystyrene or general purpose polystyrene, is typically a rather brittle resin having poor impact strength. The impact strength of polystyrene can be greatly improved by the addition of rubbery particles dispersed throughout the polystyrene resin. Polystyrene resins having improved strength achieved by addition of rubbery particles are often referred to as high impact polystyrene (HIPS), or rubber-modified polystyrene. The size of the rubbery particles and the concentration of rubber particles dispersed within the HIPS resin are believed to affect the impact strength of the HIPS resin.
The addition of the rubbery particles to polystyrene to form the HIPS resin tends to result in a reduction of the aesthetic properties of the HIPS resin and the products made from the HIPS resin. As an example, the lack of gloss of conventional HIPS resins is often a disadvantage relative to materials such as acrylonitrile-butadiene-styrene (ABS) resin, as ABS resin generally has both high impact strength and high gloss. Many consumer products require a balance of both gloss and impact strength. Examples of such products include telephones, housewares, refrigerator parts, lawn and garden tools, refrigerator liners, kitchen appliances, electronics housings, computer housings and components, water cooler housings, television housings and screens, vacuum cleaners, microwave oven doors, toys, window pieces, packaging articles, beverage containers, medical parts, etc.
Others have attempted to use other types of rubbery materials, such as styrene-butadiene block copolymers, in the preparation of HIPS resins to increase the gloss of the HIPS. Nevertheless, the impact strength of HIPS was decreased at the same time as the gloss improved. It is generally accepted that the impact strength of the HIPS resins is related to the rubbery nature of the rubbery particles. The most commonly accepted measure of this rubbery nature of the rubbery particles is the glass transition temperature, Tg, of the rubbery particles. It is also generally accepted that HIPS polymers with higher impact strengths are produced with rubbery particles having low glass transition temperatures. When styrene-butadiene block copolymers are used in the preparation of HIPS, the glass transition temperatures of the styrene-butadiene block copolymers are generally much higher than the rubbery particles due to the presence of the high Tg styrene content within the styrene-butadiene block copolymers. For example, the Tg's of polybutadienes are in the range of about −90° C. to about −110° C., whereas the Tg of polystyrene is about 100° C. Generally, if a styrene-butadiene block copolymer is used to produce a HIPS resin, a styrene-butadiene block copolymer with a minimum amount of styrene, normally less than 50 wt. % monovinylarene, is used.